1. Field
The present disclosure relates generally to methods and apparatus for spectrum sensing of signal features in a wireless channel, and more specifically to spectrum sensing of narrowband signal features using spatial diversity.
2. Background
Cognitive Radio technology, such as that employed in IEEE 802.22 Wireless Regional Area Networks (WRANs), offers the opportunity to utilize spectrum that is licensed to primary users. This unused spectrum is often referred to as “white space” spectrum. One of the primary methods of identifying white space spectrum is using spectrum sensing technology. A spectrum sensing observes the spectrum for a period of time and then determines if the spectrum is occupied by a primary user or is available for use by the cognitive radio network. A spectrum sensor must sense signals that are weaker than the noise power level. Since the signal power is so low one cannot typically decode the signal and hence must sense for signal features.
In the television band, where cognitive radio technology is likely to be first permitted, there are three primary signal types that need to be sensed: digital TV, analog TV and wireless microphones. In the US the DTV standard is ATSC and the analog TV standard is NTSC. Sensing of ATSC, NTSC and wireless microphones often comes down to sensing narrowband signal features. In ATSC one of the best signal features is the sinusoidal pilot tone. In NTSC two of the best features are the audio and video pilots. So for ATSC and NTSC, which are both 6 MHz wide, the features used in sensing are often quite narrowband. A wireless microphone has a typical bandwidth of less than 100 kHz, so it is a relatively narrowband signal.
Sensing of narrowband signal features, however, is susceptible to Rayleigh fading. Rayleigh fading causes narrowband signals to fade as much as 20 dB or more, which makes them difficult to sense. Also, the sensing performance improves slowly with increasing signal-to-noise ratio (SNR) in a Rayleigh fading channel, unlike the situation in a non-fading channel where sensing performance improves much quicker with increasing SNR.
Additionally, previous sensing techniques have utilized only a single sensing antenna, such as sensing ATSC or wireless microphones with a single sensing antenna. These known sensing techniques require estimation of the power spectrum over a portion of the channel bandwidth and, hence, suffer from the effects of Rayleigh fading. Accordingly, it would be desirable to perform spectrum sensing of narrowband features in a wireless channel with less susceptibility to Rayleigh fading in order to increase sensing performance.